This invention relates to a method of exploiting deep set, porphyry ore bodies by in situ mining techniques.
Large deep-lying deposits of copper and nickel in the form of low grade prophyry ores are known to be located throughout various regions of the globe. A porphyry deposit is one in which the copper, nickel or uranium bearing minerals occur in disseminated grains or in veinlets through a large volume of rock such as shist, silicated limestone, or volcanic rock. Acid igneous intrusive rocks are usually in close association. The deposits are typically large tonnage but low grade and have an average copper, nickel and uranium concentration of less than about a 1% total. Minerals found in these deposits are usually sulfides, of these, chalcopyrite is the most common. There are also deep-seated deposits which contain discrete blebs containing copper, copper sulfide, or copper-nickel sulfide in association with ion sulfide. In many ores of this type, significant quantities of zeolites, layered silicates, and clay such as the type known as montmorillonite are present. These minerals are present as deposits located in or about the natural microscopic fracture openings of the rock.
In the method of in-situ mining disclosed in U.S. Pat. No. 4,116,488 to Hsueh et al, an access well is drilled to communicate with the ore body and several recovery wells are provided, spaced apart from the access well. The leaching interval, i.e., the volume of rock through which leaching fluids flow between the access and recovery wells, is then subjected to fluids such as oxygen which oxidize the copper and nickel sulfides or chalcopyrite to sulfates. Thereafter, or in some cases simultaneously, an aqueous ammoniacal leach liquor is injected into the access well which, in passing through the leaching interval and contacting the metal sulfates, leaches the metal values as nickel and copper-ammonia complex ions. The leaching fluids may be injected in the form of a two-phase lixiviant, i.e., oxygen bubbles dispersed in an ammoniacal leach liquor, or may be passed sequentially through the leaching interval. One advantage of this technique is that the rock need not be fractured by explosive methods prior to leaching. Instead, the fluids used are forced through the natural fracture openings present in the rock, which typically range in the diameter between about 30 and 300 microns.
For the most efficient use of the lixiviant, it should pass rather uniformly through the leaching interval, so that its potency is not squandered on a few highly permeable passages. Igneous rock does have permeability variations, however, that cause non-uniform flow of the lixiviant through the leaching interval.
An igneous rock deposit having a permeability of 1 to 5 md may be economically mined by the in-situ method, but in such a deposit rock zones having a permeability of 25 to 50 md represent thief zones-zones of relatively high permeability that accept inordinately high amounts of lixiviant to the detriment of the over-all efficient use of the lixiviant in the process.
One technique that has been used in the petroleum industry to smooth flow involves impairing thief zones with solid particles, so that they cannot accept lixiviant so readily. However, utilizing solid particles small enough to fit into the pores of rock having a permeability of 25 to 50 md is not practical.
Furthermore, when the wellbore injection interval is several thousand feet, it can be very time-consuming and expensive to use a preliminary liquid solution to separately treat short intervals of variable permeability.
During such in situ mining efforts, the presence of clays, zeolites, and layered silicates in or about the fracture openings present problems which heretofore significantly diminished the economic feasibility of this type of process. Such minerals absorb copper and nickel as well as other ions such as uranium ions by ion exchange and are capable of taking up as much as 1.5 milliequivalents of copper per gram of ion exchanger. In practice, this uptake of metal represents perhaps as much as 25% of the total metal leached, and thus leads to significantly reduced metal recoveries. It has been discovered that the copper, nickel and uranium ions are sorbed by an in situ ion exchange process wherein calcium or other ions naturally present in the mineral are exchanged for copper, nickel or uranium ions or ammonia complexed ions such as Cu(NH.sub.3).sub.4.sup.++ or Cu(NH.sub.3).sub.3 OH.sup.+. The presence of the clays, e.g., montmorillonite clays, such as Fuller's earth and bentonite, and other absorptive minerals thus cause the uptake of significant quantities of solubilized metal values which would otherwise be recoverable.
In addition to this complication, the presence of these minerals seriously inhibits the rate at which leach liquors may be pumped through a leaching interval during in-situ mining. Also, calcium and other ions are introduced into the lixiviant thereby complicating copper, nickel and uranium recovery at the surface plant. Obviously, whether such deposits can be economically exploited hinges on whether methods can be devised which maximize metal yield, minimize reagent costs, and overcome the problems set forth above.